McNamee now heads the Energy Department's Office of Policy and worked on a stalled effort to bail out struggling coal and nuclear plants.

In the video, McNamee says fossil fuels and nuclear energy "keep the lights on" and that renewable energy such as wind and solar power "screws up ... the physics of the grid" providing the nation's electricity.

Sen. Joe Manchin, D-W.Va., opposed McNamee after backing him in committee. Manchin, whose state has long relied on coal, is one of the Senate's strongest supporters of the coal industry, but he said in a statement that McNamee wasn't serious enough about climate change.

"After viewing video footage, which I had not previously seen, where Bernard McNamee outright denies the impact that humans are having on our climate, I can no longer support his nomination," Manchin said.

"Climate change is real, humans have made a significant impact, and we have the responsibility and capability to address it urgently," Manchin added.

Some critics called Manchin's vote an effort to curry favor with environmentalists who oppose Manchin's efforts to become top Democrat on the Senate Energy and Natural Resources Committee. Sen. Maria Cantwell of Washington state is the ranking Democrat now. She is expected to become top Democrat on the Senate Commerce, Science and Transportation Committee when the new Congress convenes in January.

Manchin is one of the Senate's most conservative Democrats, and environmental groups have rallied against his promotion on the energy panel.

"Joe Manchin is the single worst possible choice to lead Senate Democrats' work on energy policy," said Josh Nelson, co-director of CREDO Action, a network of more than 5 million activists that has started an online petition to block Manchin. More than 66,000 people had signed the petition as of Thursday.

"One good vote does not make up for a career of siding with the coal industry over people and the planet," Nelson said.

The committee's top Democrat "should be a clean energy advocate with a strong track record of standing up to the fossil fuel industry," Nelson said, and not a senator "in the pocket of the coal industry."

Manchin famously shot a climate change bill with a rifle to demonstrate his opposition to the legislation.

Even so, Manchin said in his statement that climate change must be addressed, adding that he continues to believe "science and technology will be critical to ensuring that the United States continues toward a clean energy economy."

Manchin said he hopes McNamee will be open to considering the impacts of climate change and incorporate that into his decision-making at FERC, which oversees the power grid, interstate pipelines and other projects.

A spokesman for Manchin declined to comment on the petition or criticism from environmental groups.